This invention generally relates to a pretreatment process for liquefied petroleum gas. One of the products of fluid catalytic cracking (FCC) is liquefied petroleum gas (LPG). LPG, which generally comprises 3 and 4 carbon alkanes and alkenes, often contains impurities in the form of sulfur containing compounds such as hydrogen sulfide and mercaptans. In the oil refining industry, it is conventional practice to treat sour hydrocarbon streams to remove these sulfur species. Extraction processes are typically used when treating light hydrocarbons for hydrogen sulfide and mercaptan removal. Mercaptans have traditionally been removed from hydrocarbon streams because of their malodorous scent and contribution to product total sulfur.
In a typical configuration, a liquid LPG stream is fed to an amine absorber column to be contacted with an amine, such as diethanolamine (DEA) or methyldiethanol amine (MDEA), to absorb acid gases such as hydrogen sulfide and carbon dioxide. The LPG lean of hydrogen sulfide and other acid gases is then sent on for further treating to remove mercaptan sulfur compounds before it is sent to product storage or a downstream processing unit.
LPG derived from high severity, high propylene FCC units contains higher concentrations of diolefins and other impurities (such as oxygenates), which have been found to contribute to foaming in the amine absorber. Removal of these materials is envisioned as a pretreatment solution to eliminate these impurities and resolve foaming issues that would otherwise occur during the amine absorption process. However, modification of the upstream process can have a strong impact on the overall process and on the operating costs.
Therefore, a process is needed to remove impurities upstream of the amine absorber without disturbing the overall LPG treating operation.